A key characteristic of HSDPA is the use of shared-channel transmission, which implies that a certain fraction of the total downlink radio resources available within a cell, channelization code sets, and transmission power may be seen as a common resource that is dynamically shared between users, primarily in the time domain. The use of shared channel transmission on a downlink shared channel (DSCH), which in WCDMA and other systems is implemented through the High-Speed Downlink Shared Channel (HS-DSCH), enables the possibility to rapidly allocate a large fraction of the downlink resources for transmission of data to a specific user. The HS-DSCH may be associated with a group of channelization code sets. Each such channelization code set is also known as a High-Speed Physical Downlink Shared Channel (HS-PDSCH). When a base station (e.g., a NodeB or other base station) has data to send to a particular UE, the NodeB must first schedule the UE (i.e., allocate to the UE an HS-PDSCH) and then use the allocated HS-PDSCH to transmit the data to the UE. Dynamic allocation of the HS-PDSCH for transmission to a specific user maybe done on a 2 ms transmission-time-interval (TTI).
Downlink control signaling is necessary for the operation of HS-DSCH. For example, the identity of the UE that is being scheduled must be signaled to the UE along with an identification of the HS-PDSCH so that the UE will know that the base station will soon be transmitting data for the UE and will know the physical channel (e.g., code set) that is being used to send the data. The UE also needs to be informed about other parameters (e.g., the transport format used for the data transmission as well as other information). This downlink control signaling is carried on the High-Speed Shared Control Channel (HS-SCCH), which may be transmitted in a parallel to the HS-DSCH using a separate channelization code set. The HS-SCCH is a shared channel.
Referring now to FIG. 1, FIG. 1 shows an example message exchange between a NodeB 104 and a UE 106 in a HSDPA system 100. As shown in FIG. 1, the NodeB 104 transmits a pilot signal on a common pilot channel (e.g., the CPICH). The UE receives the pilot signal and uses it to compute channel quality information (CQI) and a precoding channel indicator. This information along with other information (e.g., hybrid automatic repeat request (HARQ) acknowledgement/negative acknowledgement (ACK/NAK)) is reported to the NodeB using, for example a high speed dedicated physical control channel (HS-DPCCH). The NodeB then schedules the UE for a downlink transmission and decides on the parameters for the transmission including: the HS-PDSCH, modulation and rank information (RI) (number of transport blocks), and precoding weight information (also known as precoding index (PCI)). This control information is sent to the UE using the HS-SCCH. After the control information is transmitted using HS-SCCH, the data is transmitted using the selected HS-PDSCH.
Requirements on different parts of the control information that need to be available to the UE has affected the detailed structure of the HS-SCCH. For UE complexity reasons, it is beneficial if the channelization code set is known to the UE prior to the start of the data transmission on the HS-DSCH. Otherwise the UE would have to buffer the received signal prior to dispreading or, alternatively, despread all potential HS-DSCH code sets. On the other hand, the transport block size and other information are only needed at HS-DSCH decoding/soft combining, which usually does not start until the end of the HS-DSCH TTI. Thus, the HS-SCCH information is split into two parts, part 1 and part 2. For a 2 branch multiple-input and multiple-output (MIMO) system, part 1 consists of 12 bits. These 12 bits convey information about: the channelization code set (CCS) (7 bits), the modulation scheme (3 bits), and the precoding weight information (PCI) (2 bits). RI is implicitly informed through the modulation information (MI) (i.e., the modulation scheme). Part 2 consists of 36 bits, out of which 6 bits for each transport block, 4 bits for HARQ process, 4 bits for redundancy version for the two streams and 16 bits for the identity, ID, of the UE. For single stream transmission, only 28 bits are needed for part 2 information.
The HS-SCCH coding, physical channel mapping and timing relation to the HS-DSCH is illustrated in FIG. 2. With the scheme shown in FIG. 2, the part 1 information can be decoded after one slot of the HS-SCCH subframe.
Current work within the third generation partnership project, 3GPP, regarding HSDPA evolution include addition of several new features in order to meet the requirements set by the International Mobile Telecommunications Advanced, IMT-A. The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch MIMO, i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beam forming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high signal to noise ratio, SNR, users and improves the coverage for low SNR users.
Introduction of four branch MIMO will require a new control channel structure to send the downlink grant information to the UE. Since four branch MIMO requires more bits for reporting RI and PCI this means more power is required for the control channel. However, more power to control channel degrades the performance of the HS-PDSCH, and hence degrades the system throughput.